According to one of prior arts (for example, Japanese Patent Publication No. 2005-291030), an engine operation is forcibly changed to an idling operation when an electronic control unit determines that an acceleration pedal and a brake pedal are respectively operated by predetermined amounts at the same time, in order to prevent an over-drive of a vehicle in a case that a vehicle driver accidentally steps on the acceleration pedal and the brake pedal at the same time.
According to another prior art (International Patent Publication (published in Japan) No. H2-502558), a safety circuit is provided so that an output of a driving source is changed to a predetermined smaller value when a brake pedal and an acceleration pedal are operated at the same time. According to the above prior art, the safety circuit will not be operated when it is detected, based on a differential value with time of an output from an acceleration sensor, that a vehicle driver newly stepped on the acceleration pedal, even in a case that a control unit determines that the brake pedal is operated.
According to a further prior art (U.S. Pat. No. 6,881,174), in a case that an acceleration pedal and a brake pedal are operated at the same time, an engine output is suppressed only when a required braking amount based on an operational amount of the brake pedal is larger than a required accelerating amount based on an operational amount of the acceleration pedal.
The inventors of the present invention have been conducting researches for determining whether there may be any malfunction in a control system for a driving source, such as an internal combustion engine, an electric motor and so on, which is/are mounted in a vehicle. For example, a required output value for a malfunction diagnosis is calculated based on an acceleration opening degree, which is detected by an acceleration sensor. Then, a malfunction determining threshold is set based on the above required output value for the malfunction diagnosis, and such malfunction determining threshold is compared with an estimated value of an output of the driving source (an estimated value of an actual output), so as to determine whether there may be any malfunction in the control system for the driving source. The inventors have found out new problems to be solved in the course of the above researches.
In a control system for the driving source, for which an output limiting control is carried out for limiting an output of the driving source when an acceleration pedal as well as a brake pedal is stepped on at the same time, it is possible to use a malfunction determining threshold which is calculated in the same manner to a situation in which the output limiting control is not carried out. However, in such a case, the malfunction determining threshold (which does not come under the influence of the output limiting control) is set to the estimated value of the output, which comes under the influence of the output limiting control. Therefore, it is not possible to set an appropriate malfunction determining threshold with respect to the estimated value of the output during the output limiting control. As a result, if any malfunction occurs in the control system for the driving source during the output limiting control is carried out, a time period may become longer, in which the estimated value of the output has exceeded the malfunction determining threshold and the malfunction is finally detected. Namely, a time period may become longer, in which a fail safe transaction will be carried out.